Optical-quality eyewear requires good optical performance. In the selection of optical element materials for use in optical-quality eyewear, the color, weight, and safety of the material is important, as well as good optical performance. Most often, however, the respective properties of different materials necessitate trade-offs among the desired characteristics of the optical element. For instance, glass has very good optical properties, but it is heavy (a dense material) and only impact resistant if thick (resulting in an even heavier lens).
Polymeric thermosetting resins, such as CR-39®, are lighter in weight but are lacking in impact resistance. Polycarbonate, in contrast, is both lightweight and highly impact resistant. Polycarbonate also has a high refractive index. Thus, thinner optical elements can be made utilizing polycarbonate. However, due to lower Abbe Number, polycarbonate exhibits more chromatic aberration than glass, typically resulting in unacceptable off-axis distortion. Moreover, polycarbonate is not resistant to common chemicals such as acetone or gasoline and it is sensitive to the stress cracking phenomenon.
Polyurethane materials have also been used as a material for optical elements. U.S. Pat. Nos. 5,962,617 and 6,127,505 which are herein incorporated by reference, describe a polyurethane resin material for use in lenses, visors and other optical parts. As described in these documents this material may offer improved lens characteristics over conventional materials such as impact resistance, high heat distortion temperature, lightness, very good optical properties, resistance to chemicals and to stress cracking. This material generally comprises the reaction product of a polyisocianate pre-polymer material (component A) and a diamine curing agent (component B). These two components are typically mixed just before the casting procedure and admitted into a mold cavity. Because of the very quick gel time, the casting procedure has to be completed within 30 seconds or less and therefore the feeding flow rate has to be tuned accordingly in order to admit the proper amount of material. For example, the amount of material needed for an eyewear lens is typically between 10 g for a piano lens and 40 g for an ophthalmic lens. A thermal polymerization cycle is then needed to achieve the final properties of the polyurethane material.
Conventional optical elements, for example sunglass lenses manufactured using the aforementioned polyurethane materials, however, fail to properly reduce the effects of glare (i.e. the presence of areas or “hot spots” in the field of vision which are of sufficient brightness to cause visual impediment, such as temporary blurring of vision, or ocular fatigue), since they reduce uniformly the intensity of light throughout the visible spectrum. For example, if a hot spot is ten times brighter than the background ambient light, it will remain ten times brighter if a conventional optical element is used to reduce transmittance of the light, say by 50%. Therefore, conventional optical elements do not significantly eliminate the discomfort to the eye or the blurred vision resulting from the difference in the light intensity between the hot spot and the background.
In order to obviate to this problem, the use of polarized optical elements has been suggested in the art.
Polarized optical elements, in fact, can reduce glare by optically filtering the polarized light significantly more than the non-polarized light. Directly reflected sunlight is partially polarized while ambient light is not, and therefore the transmittance of the reflected sunlight would be reduced much more than that of the ambient light, thereby reducing the discomfort to the eyes.
Generally, polarized optical elements such as polarized lenses are obtained by bonding a polarized film onto the plastic surface of the lens substrate or by introducing such a film into the plastic material of the lens substrate during polymerization or by applying a polarizing coating on a lens substrate. These methods, as well as alternative methods of producing polarized lenses are disclosed in U.S. Pat. No. 6,650,473. the content of which is herein incorporated by reference. Regardless of which particular material is used for the substrate of the optical element, it is preferable in many applications to incorporate a polarizing film into the optical element.
Suitable polarizing films may comprise a variety of different constructions and materials. Such constructions include freestanding or non-laminated polarized films, such as polyvinyl alcohol (PVA) or polyethylene terephthalate (PET) films, films with removable protective sheeting, and films with outer permanent protective coatings or supportive plastic coatings.
Polyurethane, impact-resistant polarized lenses incorporating a polarizing film are described in US patent applications No. 2001/0028435 and 2004/0021941.
The polyurethane lens-forming material described in these references is preferably a material disclosed in U.S. Pat. No. 5,962,617, the content of which is incorporated herein by reference, or a modification thereof, obtained by a procedure called the “full-prepolymer method”. The full-prepolymer method essentially consists in preparing a prepolymer containing free —NCO functional groups by reacting a diisocyanate with a slightly less than stoichiometric amount of long and/or short chain polyols and then mixing the prepolymer thus obtained (component A) with a curing agent, such an aromatic amine or a short chain polyol. (component B) which possesses functional groups capable of reacting with the free —NCO functional groups of the prepolymer. This intermediate material is then admitted into a mold made of an appropriate material such as glass or metal, and left to cure by means of a suitable time/temperature cycle.
According to experiments carried out by the present inventors and as confirmed by US patent application No 2001/0028435, however, the attempts to combine a high impact polyurethane polymeric material with a standard PVA polarized film gave negative results, since the film was consistently displaced and bent out of shape during the introduction of the polymeric material. Only polarizing films made of freestanding PET film or wafers comprising transparent plastic sheets laminated on at least one side of a PVA film to impart the PVA a sufficient robustness were used with success.
According to the inventors of US patent application No 2001/0028435, the reason for these negative results would be that it is very difficult to consistently achieve with high yields polarized polyurethane impact-resistant lenses with an embedded freestanding PVA film because during the introduction of the lens-forming resin the PVA film can be displaced due, at least in part, to the quick setting time of the resin itself. Moreover, the inventors of this reference believed that the heat produced by the exothermal reaction during the gelification of the polyurethane resin composition can soften the PVA film making easier the displacement of the film itself.
U.S. Pat. No. 6,734,272 discloses a polarized optical lens obtained from a polyurethane resin composition which is said to have a good resistance to discoloration and durability and a long pot life. According to this reference, in order to take a sufficient time for casting work and overcome the problem related to the short pot life of the mixture of the prepolymer obtained by reacting a polyisocyanate with a polyhydroxy compound (component A) and of the aromatic polyamine (component B), 3,3′-dichloro-4,4′-diaminodiphenylmethane also known as 4,4′-methylene-bis(2-chloroaniline) (MOCA) is used as diamine curing agent (component B). From a practical point of view, however, the use of this diamine curing agent is very undesirable since MOCA is recognized as a very carcinogenic substance and therefore severe health and safety issues are involved. Additionally, resins produced using this diamine curing agent have a very high yellowness, which is another undesirable property.
Accordingly, a need still exists of overcoming the fundamental difficulty of handling the high impact polymeric material and of reproducibly positioning a polarizing film within the final optical element, achieving at the same time adequate optical and mechanical characteristics of the optical element.